Boko Haram militants have killed a female aid worker held by the group since March, the Nigerian government has confirmed. The aid worker was shot in the head, execution style after a deadline imposed to release imprisoned members of the armed group expired, the government said.

The Nigerian government said it was "shocked" and "saddened" by the killing of the woman, the information ministry said in a statement.

A clip of the execution of 24-year-old Hauwa Leman was seen on Monday by a correspondent of Nigeria's online news platform, The Cable. "Leman was forced to kneel down, with her hands tied inside a white hijab which has a crest symbol, and then shot at a close range," The Cable reported.

Leman, a nurse with the International Committee of the Red Cross (ICRC), was kidnapped by the armed group in March along with two other medical workers in Rann, Borno state in the country's northeast. One of the workers - Saifura Ahmed, who had been abducted at the same time, was executed by Boko Haram in September. Boko Haram has made the country’s northeast its stronghold and regularly carries out attacks, raids and kidnappings.

"We have kept our word exactly as we said, by killing another humanitarian worker, Hauwa Leman, who is working with the International Committee of the Red Cross (ICRC) that were abducted during a raid on a military facility in Rann, Kala Balge in March 2018," Boko Haram said in a short statement sent to the Cable.

"Saifura and Hauwa were killed because they are considered as Murtads (apostates) by the group because they were once Muslims that have abandoned their Islam, the moment they chose to work with the Red Cross, and for us, there is no difference between Red Cross and UNICEF," the group added.

"If we see them, we will kill the apostates among them, men or women, and chose to kill or keep the infidels as slaves, men or women."

An ICRC spokeswoman told Reuters on Monday, "We are hearing devastating reports that Hauwa has been executed. At this stage, we have no confirmation that this is true. We desperately hope not. This situation is heartbreaking and our thoughts remain with the family."